How to make Image as Planes default to Diffuse shader instead of principled BSDF to in 2.8x
On the window that opens when you add "import images as planes", there is a gear icon on the top right. Click on it and you will quite a few options open for you.  The shortcut, as you might have guessed already, is N

By default, the material is set to use the the BSD principled shader, but you can change it to Shadeless or Emit.

It is there that you can choose to use Alpha channel.

If you have a series of pictures you an choose to load all of them with a certain offset.

Lastly you can set the size and orientation.

 

For images that are already imported just change the principled shader for whatever it shader you like, and blend happily ever after.



EDIT:

Comparing the files from the previous version (2.79) of the addon it is quite easy to modify it to use a Diffuse shader.



Find the io_import_imates_as_planes.py file located in the /scripts/addons folder, open it in a text editor and replace all mention of Principled with Diffuse, make sure you match Upper and Lowercase correctly.

Then on lines 1013 and 1014 replace

     if self.shader == 'PRINCIPLED':
        core_shader = node_tree.nodes.new('ShaderNodeBsdfPrincipled')


with

   if self.shader == 'DIFFUSE':
        core_shader = node_tree.nodes.new('ShaderNodeBsdfDiffuse')


on line 1024

replace

    if self.use_transparency:
        if self.shader == 'PRINCIPLED':
            node_tree.links.new(core_shader.inputs[18], tex_image.outputs[1])
        else:
            bsdf_transparent = node_tree.nodes.new('ShaderNodeBsdfTransparent')

            mix_shader = node_tree.nodes.new('ShaderNodeMixShader')
            node_tree.links.new(mix_shader.inputs[0], tex_image.outputs[1])
            node_tree.links.new(mix_shader.inputs[1], bsdf_transparent.outputs[0])
            node_tree.links.new(mix_shader.inputs[2], core_shader.outputs[0])
            core_shader = mix_shader


with

    if self.use_transparency:

        bsdf_transparent = node_tree.nodes.new('ShaderNodeBsdfTransparent')
        bsdf_transparent = node_tree.nodes.new('ShaderNodeBsdfTransparent')

        mix_shader = node_tree.nodes.new('ShaderNodeMixShader')
        node_tree.links.new(mix_shader.inputs[0], tex_image.outputs[1])
        node_tree.links.new(mix_shader.inputs[1], bsdf_transparent.outputs[0])
        node_tree.links.new(mix_shader.inputs[2], core_shader.outputs[0])
        core_shader = mix_shader


Be aware of the indentation change, as you are getting rid of a conditional if and else statements.

Save the file and enable the addon again.

Here's a version of the modified file